This application addresses a fundamental question of significance to understanding human nature and the entire field of social neuroscience: "What distinguishes a true social interaction between human beings from other perceptuo-motor interactions?" Understanding how coordination occurs both within the human brain and between human brains is vital to the basic and clinical neuroscience mission of NIMH, The reason is that disruptions of coordinative interactions among cortical and subcortical areas and the breakdown of neural integration are thought to lie at the heart of major neuropsychiatric disorders such as schizophrenia and autism. We aim to 1) identify the neurobehavioral mechanisms underlying how brain regions couple and decouple both within an individual brain and between brains engaged in social settings and 2) clarify the nature of the informational coupling between individuals engaged in social behavioral interactions. The work will be carried out by a dedicated interdisciplinary team of researchers whose expertise spans cognitive neuroscience, psychology and physics. It builds upon and explores in depth our recent discovery of an oscillatory complex in the brain that may act as a specific neuromarker of social behavior. The proposed research uses behavioral methods and sophisticated computational analyses in combination with dual high density EEG electrode arrays developed for the simultaneous recording of brain activity from interacting pairs of people to uncover where social signals are processed in the brain and how they become coupled dynamically between individuals. The novel conceptual and analytic framework of coordination dynamics is used to explore cognitive (within brain) and social (shared between-brain) neural representations hypothesized to underlie social coordination. Coordination dynamics provides theoretical concepts, methods and ecologically valid paradigms to attack the outstanding question of how large scale integration in the brain and across brains is accomplished. The research represents a first attempt to incorporate dynamical systems analysis into the study of socially-mediated neurophysiological activity in conscious, interacting humans. Uncovering the neurobiological mechanisms that underlie coordinated human behavior in an individual during social interactions constitutes a major step toward understanding neuropsychiatric disorders and is likely to result in the development of more effective pharmacological and behaviorally relevant treatments.